I'm almost finished, and this game does get crazy difficult at times. I couldn't recommend it more (you can guess that from the post above).
What's really striking is how the game designer(s?) has managed to teach core skills through play. Again and again you run into situations where you have to figure out how the world and it's mechanics works to be able to progress. If you have ever seen the valve people talk about portal, or the gravity gun from Half-Life 2, you know that they focus a lot on learning through doing, and this game does exactly that, just a lot better.
The game is also beautiful, and sounds great, and the posters that's everywhere are always funny and often thought-provoking. All in all such a great experience.